Dead fish piled up in mangrove roots off of Sykes Creek. Residents started noticing a fish kill in Sykes Creek on April 10, 2018, a current kept the fish from piling up. There were also reports of fish near the surface struggling for air in the Banana River.(Photo: MALCOLM DENEMARK/FLORIDA TODAY)Buy Photo

Puffers, mullet, catfish and other scaly carcasses dotted the surface of northern Sykes Creek on Tuesday, reminiscent of the Indian River Lagoon's worst known fish kill two years ago.

A state fish kill hotline this week fielded about a dozen reports, citing from 25 to 50 puffer fish, red drum, sheepshead, mullet, catfish and flounder in the Sykes Creek area near Ulumay Sanctuary. And fish have also been seen gasping for air this week in the Cocoa Beach area, said Capt. Alex Gorichky of Local Lines Charters on Merritt Island.

Fish kills often defy predictions, biologists say. But, with more cloudy weather on the way this week, prospects have grown of another mass fish die-off, like the "fish-pocalypse" the Space Coast saw in 2016.

"I wouldn't be surprised to see it get worse," said Duane DeFreese, executive director of the Indian River Lagoon National Estuary ​Program. "It's very hard to predict."

Dissolved oxygen is among the most vital indicators of good water quality, essential for fish and other marine life. Fish need at least 2 milligrams per liter of dissolved oxygen in the water. Below that, they begin to suffocate.

During daylight hours, algae produce oxygen through photosynthesis, replenishing oxygen levels in the water. But, at night, the algae consume oxygen. This, coupled with the normal demand for oxygen from fish, crabs and other marine life can cause dips in dissolved oxygen in the lagoon, with the lowest levels just before dawn.

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No filter: This is a photo of the canal parallel to Minutemen Causeway looking east through the mangroves from the bridge at Sunset Road on April 26, 2018. Officials think the red color may be caused by a Fibrocapsa japonica bloom. MALCOLM DENEMARK/FLORIDA TODAY

Reports of an algae bloom in the canal between Sunset Road and Aucilia Road in Cocoa Beach started April 23, 2018, with the canal having a very red tint to it. Some suspected an algae boom while others suspected a spill. The reddish color was only near the canals near the two streets. MALCOLM DENEMARK/FLORIDA TODAY

Reports of an algae bloom in the canal between Sunset Road and Aucilia Road in Cocoa Beach started April 23, 2018, with the canal having a very red tint to it. Some suspected an algae boom while others suspected a spill. The reddish color was only near the canals near the two streets. MALCOLM DENEMARK/FLORIDA TODAY

A dolphin swims in the brown water of Sykes Creek. Residents started noticing a fish kill in Sykes Creek on April 10, 2018, a current kept the fish from piling up. There were also reports of fish near the surface struggling for air in the Banana River. MALCOLM DENEMARK/FLORIDA TODAY

A dead fish floats by the fishing pier in Sykes Creek. Residents started noticing a fish kill in Sykes Creek on April 10, 2018, a current kept the fish from piling up. There were also reports of fish near the surface struggling for air in the Banana River. MALCOLM DENEMARK/FLORIDA TODAY

A puffer fish floats by the fishing pier in Sykes Creek. Residents started noticing a fish kill in Sykes Creek on April 10, 2018, a current kept the fish from piling up. There were also reports of fish near the surface struggling for air in the Banana River. MALCOLM DENEMARK/FLORIDA TODAY

Dead fish piled up in mangrove roots off of Sykes Creek. Residents started noticing a fish kill in Sykes Creek on April 10, 2018, a current kept the fish from piling up. There were also reports of fish near the surface struggling for air in the Banana River. MALCOLM DENEMARK/FLORIDA TODAY

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Cloudy days limit how much oxygen algae and other aquatic plants can produce during the day, increasing risk of fish kills.

Cooler weather could somewhat temper the fish-kill threat, however. The warmer water gets, the less oxygen it can hold. In warmer waters, there are larger spaces between water molecules, so it's easier for oxygen to escape. Water molecules pack closer together in colder water, holding oxygen tighter.

When dense clusters of fish die and rot, bacteria increase, further diminishing the available oxygen in the water.

“I wouldn't be surprised to see it get worse.”

Duane DeFreese, executive director, Indian River Lagoon National Estuary ​Program

Winds could help aerate the water, but stagnant days pose more of a threat.

Real-time water monitors dipped as low as 3.5 milligrams per liter this week.

Heavy rains can add another stress to fish, crabs and other lagoon life by lowering salt levels in the water near the threshold of what marine creatures need to survive.

Recent satellite images of the lagoon's algae blooms have resembled similar images from the spring 2016 algae blooms. The images show levels of chlorophyll — an indicator of algae blooms — remain high in much of Brevard's portion of the lagoon, due to the mix of single-celled algae, mostly the species responsible for brown tide.

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A dolphin swims in the brown water of Sykes Creek. Residents started noticing a fish kill in Sykes Creek on April 10, 2018, a current kept the fish from piling up. There were also reports of fish near the surface struggling for air in the Banana River.(Photo: MALCOLM DENEMARK/FLORIDA TODAY)

In 2016, fish died from Titusville to Palm Bay in what at the time was described as the worst lagoon fish kill on record.

DeFreese said it is important for residents and others who see dead fish to report them to the state's Fish Kill Hotline: 800-636-0511.

This provides Brevard County officials with early warning signs and information about where to focus cleanup efforts.

"If it's just a few fish, we won't be mobilizing to pick up fish, but if it gets to 2016 (levels),
there are plans at the county level, with state support, to start responding," DeFreese said. "We're in wait-and-see mode."

People also can report fish kills on the state's new smartphone application, called "FWC Reporter." To find the application, go to your smartphone app store and search for "FWC Reporter."

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Residents in the Cocoa Isles neighborhood in Cocoa Beach started seeing evidence of a fish kill in the Banana River on Saturday, with hundreds of dead, bloated fish floating in the river or washing up along the shore near the Thousand Islands. MALCOLM DENEMARK/FLORIDA TODAY

Residents in the Cocoa Isles neighborhood in Cocoa Beach started seeing evidence of a fish kill in the Banana River on Saturday, with hundreds of dead, bloated fish floating in the river or washing up along the shore near the Thousand Islands. MALCOLM DENEMARK/FLORIDA TODAY

Residents in the Cocoa Isles neighborhood in Cocoa Beach started seeing evidence of a fish kill in the Banana River on Saturday, with hundreds of dead, bloated fish floating in the river or washing up along the shore near the Thousand Islands. MALCOLM DENEMARK/FLORIDA TODAY

Residents in the Cocoa Isles neighborhood in Cocoa Beach started seeing evidence of a fish kill in the Banana River on Saturday, with hundreds of dead, bloated fish floating in the river or washing up along the shore near the Thousand Islands. MALCOLM DENEMARK/FLORIDA TODAY

Residents in the Cocoa Isles neighborhood in Cocoa Beach started seeing evidence of a fish kill in the Banana River on Saturday, with hundreds of dead, bloated fish floating in the river or washing up along the shore near the Thousand Islands. MALCOLM DENEMARK/FLORIDA TODAY

Residents in the Cocoa Isles neighborhood in Cocoa Beach started seeing evidence of a fish kill in the Banana River on Saturday, with hundreds of dead, bloated fish floating in the river or washing up along the shore near the Thousand Islands. MALCOLM DENEMARK/FLORIDA TODAY

Residents in the Cocoa Isles neighborhood in Cocoa Beach started seeing evidence of a fish kill in the Banana River on Saturday, with hundreds of dead, bloated fish floating in the river or washing up along the shore near the Thousand Islands. MALCOLM DENEMARK/FLORIDA TODAY

Residents in the Cocoa Isles neighborhood in Cocoa Beach started seeing evidence of a fish kill in the Banana River on Saturday, with hundreds of dead, bloated fish floating in the river or washing up along the shore near the Thousand Islands. MALCOLM DENEMARK/FLORIDA TODAY

Residents in the Cocoa Isles neighborhood in Cocoa Beach started seeing evidence of a fish kill in the Banana River on Saturday, with hundreds of dead, bloated fish floating in the river or washing up along the shore near the Thousand Islands. MALCOLM DENEMARK/FLORIDA TODAY

Residents in the Cocoa Isles neighborhood in Cocoa Beach started seeing evidence of a fish kill in the Banana River on Saturday, with hundreds of dead, bloated fish floating in the river or washing up along the shore near the Thousand Islands. MALCOLM DENEMARK/FLORIDA TODAY

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There have been reports of fish kills all along the Indian River Lagoon this week. On Thursday, there were more than a hundred dead rays lining the shallow water just offshore of the Pineda Causeway on the southeast side. Dead puffer fish and other species floated in the river or lined the shore. MALCOLM DENEMARK/FLORIDA TODAY

There have been reports of fish kills all along the Indian River Lagoon this week. On Thursday, there were more than a hundred dead rays lining the shallow water just offshore of the Pineda Causeway on the southeast side. Dead puffer fish and other species floated in the river or lined the shore. An eel struggles to breath. MALCOLM DENEMARK/FLORIDA TODAY

There have been reports of fish kills all along the Indian River Lagoon this week. On Thursday, there were more than a hundred dead rays lining the shallow water just offshore of the Pineda Causeway on the southeast side. Dead puffer fish and other species floated in the river or lined the shore. MALCOLM DENEMARK/FLORIDA TODAY

There have been reports of fish kills all along the Indian River Lagoon this week. On Thursday, there were more than a hundred dead rays lining the shallow water just offshore of the Pineda Causeway on the southeast side. Dead puffer fish and other species floated in the river or lined the shore. A dead flounder sits on the bottom. MALCOLM DENEMARK/FLORIDA TODAY

There have been reports of fish kills all along the Indian River Lagoon this week. On Thursday, there were more than a hundred dead rays lining the shallow water just offshore of the Pineda Causeway on the southeast side. Dead puffer fish and other species floated in the river lined the shore. MALCOLM DENEMARK/FLORIDA TODAY

There have been reports of fish kills all along the Indian River Lagoon this week. On Thursday, there were more than a hundred dead rays lining the shallow water just offshore of the Pineda Causeway on the southeast side. Dead puffer fish and other species floated in the river or lined the shore. A dead puffer fish floats by a dead flounder on the bottom, with a horseshoe crab nearby. MALCOLM DENEMARK/FLORIDA TODAY

There have been reports of fish kills all along the Indian River Lagoon this week. On Thursday, there were more than a hundred dead rays lining the shallow water just offshore of the Pineda Causeway on the southeast side. Dead puffer fish and other species floated in the river or lined the shore. Charter boat captain and lagoon activist Alex Gorichky holding a dead flounder. MALCOLM DENEMARK/FLORIDA TODAY

There have been reports of fish kills all along the Indian River Lagoon this week. On Thursday, there were more than a hundred dead rays lining the shallow water just offshore of the Pineda Causeway on the southeast side. Dead puffer fish and other species floated in the river or lined the shore. Flounder and dead rays on the bottom of the river. MALCOLM DENEMARK/FLORIDA TODAY